A photoresist mask is formed on a semiconductor substrate by first coating the substrate with a thin layer of photoresist, exposing the resist with the desired pattern, and then developing the photoresist layer. In the first step, the substrate is coated with photoresist by a spin coating machine. In the spin coating machine, a liquid that includes photoresist dissolved in a solvent is applied to the top surface of the substrate while the substrate is spun slowly. During this process, a solvent in the liquid evaporates, causing the photoresist to solidify. The liquid is applied via a liquid dispense line. Ordinarily when the liquid is dispensed, there is no thermal control as it passes through the dispense line. Applicants have found that this lack of thermal control results in non-uniform coatings. Therefore, Applicants have found it necessary for liquids dispensed in a spin coating machine to be temperature controlled. Similarly, Applicants have also found it necessary for liquids dispensed in a developer machine to be temperature controlled.
FIG. 1 illustrates an exploded view of the spin coating machine which has been in use by the industry for a number of years. Spin coating machine 100 is illustrated schematically as containing a liquid dispense line 120 that dispenses liquid from a nozzle assembly (point of use) 104C on a substrate 103. Substrate 103 is supported by a flat vacuum chuck 102 connected to a spin motor 101.
Spin coating machine 100 has an exhaust system that includes a liquid drain line 105, an air purge line 106 and a safety line 107. Lines 105 and 106 connect to a coating chamber 100A and line 107 connects to a motor chamber 100B of spin coating machine 100. Lines 105 and 106 lead to a drain tank 109, which is used to collect excess liquid. Line 107 leads to an exhaust manifold 110, which is connected to the factory exhaust system. A connecting line 111 allows vapor from drain tank 109 to enter exhaust manifold 110.
As each substrate is processed, in order to prevent the liquid from drying too quickly, a selected concentration of vapor is maintained in the atmosphere in coating chamber 100A. This may be done advantageously with the arrangement described in commonly-owned copending application Ser. No. 07/904,795, U.S. Pat. No. 5,289,222 filed Jun. 26, 1992, which is incorporated by reference herein in its entirety. During the spin coating process, programmable exhaust unit 112 closes a butterfly valve (not shown) in conduit 111 so as to restrict the flow of air and vapor from drain tank 109 to exhaust manifold 110. This in turn limits the flow of vapor and air in lines 105 and 106 and maintains the vapor concentration in coating chamber 100A at an increased level. When the coating process is completed, programmable exhaust unit 112 opens the butterfly valve and the normal flow of the vapors and air through lines 105 and 106 into the factory exhaust system is reestablished.
Applicants have found several disadvantages with the prior art apparatus and method. The main disadvantage of the prior art apparatus and method is that the temperature of the liquid dispensed over the top of the substrate is uncontrolled. Therefore, variations in the temperature of the surroundings may lead to non-uniform coatings even though a selected vapor concentration is maintained by use of the butterfly valve.
Some prior art systems may control the temperature of a liquid dispense line to a limited extent. Even in such prior art systems, there is no temperature control at the liquid exit end (point of use) of the liquid dispense line and for a significant distance (several feet) upstream from the liquid exit end. The lack of temperature control over this distance results in a change in temperature of the liquid. This problem becomes especially acute if the liquid is not continuously dispensed. For example, if liquid dispensing is suspended over-night, temperature of the liquid around the liquid exit end cannot be controlled when the system is started up again.
Also, in some prior art systems that provide cooling of the dispense liquid, the coolant is circulated in a jacket around a single liquid dispense line. Since mechanical fittings are used to mount the jacket on to the liquid dispense line, such prior art fittings occasionally leak and coolant is discharged. Even though the jacket terminates at a significant distance upstream from the liquid exit end (point of use), any coolant discharge from the jacket may result in damage to the substrate or to other machinery in the system.
Finally, prior art devices used to cool the dispense liquid are generally of a size sufficiently large to block air flow to the substrate and around the substrate. Such improper blockage of air flow leads to non-uniform coatings.